1. Field of Invention
This invention relates to optical transmission systems, and more particularly to optical transmission systems having reduced Brillouin scattering.
2. Discussion of Related Art
Demand for optical communication systems is growing with the growing demand for faster broadband and more reliable networks. Wavelength division multiplexing (WDM) is one technique used to increase the capacity of optical communication systems. Such optical communication systems include, but are not limited to, telecommunication systems, cable television systems (CATV), and local area networks (LANs). An introduction to the field of Optical Communications can be found in “Optical Communication Systems” by Gowar, ed. Prentice Hall, NY, 1993.
WDM optical communication systems carry multiple optical signal channels, each channel being assigned a different wavelength. Optical signal channels are generated, multiplexed to form an optical signal comprised of the individual optical signal channels, and transmitted over a single waveguide such as an optical fiber. The optical signal is subsequently demultiplexed such that each channel corresponding to a wavelength is individually routed to a designated receiver.
In wavelength division multiplexing, the transmitted wavelengths are locked to one of the International Telephone Union (ITU) standard wavelengths, called the ITU grid, to meet cross-talk specification and reliability in operation over time. Technologies such as Distributed Feedback Lasers (DFB) are used to provide a source at a desired wavelength for the ITU grid.
In optical transmission systems it is desirable to transmit signals, at wavelength channels, via optical waveguides (optical fibers) at high power to maintain sufficient signal to noise ratios. This is particularly desired when transmitting the signals over extended distances so as to reduce the bit error rate in the received optical signal. Optical fibers, however, comprise a medium that exhibits nonlinear behavior at high power levels which results in detrimental performance of the transmission systems.
Stimulated Brillouin Scattering (SBS) within a core of an optical fiber results from photons being scattered by localized refractive index variations (acoustic grating) induced by acoustic (i.e., sound) waves or acoustic phonons. These refractive index variations are caused by acoustic vibrations in the silica lattice that makes up the core of the fiber. Due to the dependence of the refractive index on light intensity in the nonlinear regime, the high intensity light in the fiber will induce lattice vibrations which results in creation of acoustic waves that scatter more light. The optimum power level at which optical signals can be transmitted is typically the maximum power level at which degradation of the signal due to nonlinear effects is avoided. That is the threshold power at which stimulated Brillouin scattering Occurs (SBS Threshold).
When the threshold light power is exceeded (as low as 5 mW per channel depending on the quality of the optical fiber, length of fiber, and other components in the communication system), light from an intense forward propagating signal, for example light from a laser launched into an optical fiber, can interact nonlinearly with the lattice of the core material of the optical fiber to generate vibrations or acoustic phonons which in turn promote the appearance of Stimulated Brillouin Scattering (SBS) which takes the form of a backward propagating signal also known as a Stokes signal. The stokes signal is responsible for degrading the forward propagating signal thus degrading system performance as well as potentially damaging transmitter components.
One way of avoiding this problem is to limit the power of the transmitted signal (forward propagating). However, a reduction in the forward propagating signal reduces the allowable un-repeatered span length in fiber transmission systems, as well as the number of splits which can be used in a fiber distribution system such as a CATV system.
Another way of alleviating this problem is to increase the power at which the onset of SBS occurs, that is increase the SBS threshold. This threshold is defined as the level of forward optical power at which the power of the backward Stokes signal becomes equal to the power of the Rayleigh scattered signal.
Therefore, it is desirable to overcome these and other limitations thus allowing overall improved performance and/or reduced cost of the transmission network.